Certain organs of the body mechanically expand and contract on a regular basis, most notably the diaphragm and lungs during breathing and the heart as it beats. Muscle groups and body limbs also mechanically expand and contract and move under the control of the central nervous system. When a dysfunction occurs in these body systems, examinations are conducted to determine the nature of the dysfunction and a variety of therapies are prescribed to restore the function. Such examinations include monitoring and the delivery of therapies including drugs and/or electrical stimulation to restore the affected function.
With respect to the heart, the cells of the chambers of the heart relax and contract in an organized and relatively rhythmic cycle due to gradual polarization followed by rapid and organized depolarization of the cardiac cells. The depolarization is accompanied by a relatively forceful contraction of the chamber in a manner described extensively in the literature. Cycles of this type exhibit characteristic electrical signal waveforms of the polarization-depolarization-re-polarization wavefronts referred to as the PQRST electrogram complex. The accompanying relaxation of the heart muscle draws blood into the chambers and the forceful mechanical contraction ejects blood through the valves. The resulting blood pressure waves are audible through the adjacent tissue of the thoracic cavity as a characteristic "lub-dub" sound. In a similar manner, the relaxation and contraction of the diaphragm causes air to be drawn in and ejected from the lungs with a characteristic motion and sound.
For a wide variety of reasons, the detection, display and/or measurement of the electrical signals and the acoustic waves or sounds of these heart and lung cycles have long been of medical interest. In addition, the mechanical motion of the lungs and surrounding thoracic cavity have been the subject of measurement using impedance plethysmography.
Of course, cardiac sounds and the sounds of respiration are commonly listened to by trained medical personnel employing use of passive or active stethoscopes manually positioned over the patient's chest during a medical examination. In this manner, congestion in the lungs, if present, or the characteristic lub-dub sounds of the sequential depolarization of the atria and ventricles of the heart may be listened to. Together with other symptoms, a number of illnesses of the heart and lungs may be diagnosed and treatment prescribed.
Efforts have been underway for many years to develop implantable sensors for temporary or chronic use in a body organ or vessel for a variety of uses, and particularly for measuring aspects of the cardiac and breathing cycles. Catheters and leads are used in conjunction with a wide variety of cardiac medical devices, including implanted and external pacemakers, cardioverter/defibrillators, drug dispensers, cardiac monitors, cardiac assist devices, implanted cardiomyoplasty stimulators, muscle and nerve stimulators, and the like. One or more catheter or lead is attached at the proximal end thereof to a device port or terminal and the distal end segment thereof is introduced into direct contact with the heart muscle or extended within the atrial and/or ventricular heart chamber in contact with blood and in indirect contact with the heart muscle. A condition of the heart or of the blood is sensed and/or a therapy is delivered through the lead or catheter.
For example, a simple cardiac catheter for measuring blood pressure changes includes an elongated catheter body extending between a proximal connector end and a distal end having one or more end openings or a balloon adapted to be introduced into a heart chamber. Drugs or agents may be dispensed through the catheter lumen to a desired location. Blood pressure fluctuations in the column of fluid in the lumen may be measured as long as the distal end opening remains open or the balloon remains capable of flexing with pressure changes. However, unless anticoagulants are continuously dispensed through the catheter lumen, the distal end of the catheter becomes encased in fibrosis interfering with balloon motion and/or closing the end openings within a relatively short time. For this and other reasons, such simple blood pressure measuring catheters cannot be left in place chronically or implanted permanently in association with an implanted medical device.
Catheters have also been proposed including sensors incorporated into the catheter distal tip for measuring various blood parameters. In these cases, the catheter body incorporates electrical conductors and proximal end connector terminals in order to power such sensors and to convey electrical signals from the sensors to an implanted or external medical device.
A lead is a form of a catheter having one or more lead conductors extending between a proximal connector terminal(s) and distal exposed electrode(s) from which electrical stimulation may be delivered or electrical signals of the body may be detected. A pacing lead includes one or more pace/sense electrodes and associated lead conductors. A cardioversion/defibrillation lead includes one or more cardioversion/defibrillation electrodes and associated lead conductors, and may also include one or more pace/sense electrodes and associated lead conductors.
In the context of cardiac pacemakers and/or cardioverter/defibrillators comprising implantable pulse generators (IPGs) and leads of this type, a variety of indwelling sensors have been proposed in combination with the leads for sensing parameters including blood pressure, the rate of change of blood pressure, blood gas concentrations, blood pH, and blood temperature or for sensing the mechanical motion of the heart. The sensed signals as well as the P-wave and/or R-wave of the heart sensed through the pace/sense electrodes in contact with the patient's atrium and/or ventricle, respectively, are employed for a variety of reasons.
In pacemakers, such indwelling sensors have been proposed for use in algorithms for adjusting the pacing rate to meet the demand for cardiac output as related to a characteristic of the sensed parameter that varies with exercise. Other rate-responsive pacemakers have been widely commercialized employing an IPG mounted piezoelectric crystal sensor responsive to the level of patient activity, referred to as an "activity" sensor. The use of an activity sensor and indwelling sensor or two or more indwelling sensors in combination and the processing of the sensor signals to derive a pacing rate control signal are disclosed, for example, in commonly assigned U.S. Pat. No. 5,188,078.
Still other rate-responsive pacemakers have been commercialized employing impedance variations with respiration as measured between spaced thoracic electrodes as disclosed, for example, in EPO Patent Nos. 0 089 014 and 0 151 689, incorporated herein by reference. The thoracic electrodes may be spaced apart and away from the heart or may include a pacing lead electrode. In the '689 patent, a pacing rate control signal is developed from the variation in the measured impedance as a function of pulmonary minute ventilation.
The use of a variety of sensors in pacemakers is also proposed for detecting capture of the heart by a pacing pulse, i.e., detecting the evoked depolarization in response to preceding pacing pulse. In order to conserve battery power and prolong the life of implanted pacemakers, it is desirable to minimize the energy of the pacing pulse to provide a minimal "safety margin" of the pulse energy over the threshold energy sufficient to capture the heart. The detection of the evoked electrical signal is difficult because the pacemaker sense amplifier is "blinded" by the residual "polarization" energy of the pacing pulse at the sensing electrode for a time period. While the detection of an evoked ventricular depolarization R-wave superimposed on the decaying residual polarization wave may be possible under carefully controlled circumstances, the detection of the much lower amplitude evoked P-wave has not proven possible. Therefore, the alternate use of indwelling sensors of the types described above to detect the change in blood temperature, gas concentration, or pressure accompanying the evoked response has been proposed as described in detail in commonly assigned U.S. Pat. Nos. 5,320,643, 5,331,996 and 5,342,406.
In a further approach, a system disclosed in U.S. Pat. No. 4,114,628 suggests the use of a mechanical heart motion sensor in contact with the heart to detect capture or LOC. The disclosed sensor in the '628 patent is a moving core, coiled wire inductor transducer mounted within an endocardial or epicardial lead coupled to the IPG.
In the context of automatic implantable cardioverter/defibrillators, fibrillation is typically detected by the continuous detection and analysis of features of the P-waves or R-waves, including high rate, rate regularity, sudden onset of high rate, etc. When the atria or ventricles of the heart are in fibrillation, the associated P-waves or R-waves become chaotic, and the heart chamber is unable to vigorously contract in the normal manner. The confirmation of ventricular fibrillation by detecting the absence of blood pressure is proposed in U.S. Reissue Pat. No. Re 27,652. The confirmation of ventricular fibrillation by detecting the absence of heart motion characteristic of normal contractions along with a R-wave high rate has been proposed in U.S. Pat. Nos. 3,815,611 and in the above-referenced '628 patent. The use of the indwelling blood pressure sensors, blood gas concentration sensors or temperature sensors to confirm fibrillation by detecting a change in the measured parameter in conjunction with the high rate P-wave or R-wave is also suggested in the prior art.
Implantable cardiac monitors, e.g. the MEDTRONIC.RTM. implantable hemodynamic monitor employ pacing leads attached to the patient's heart for simply monitoring and storing EGM data and an indwelling pressure sensor for developing an absolute pressure signal from the heart. A reference pressure sensor is incorporated into the connector block assembly for detecting ambient atmospheric pressure that is subtracted from the pressure sensor signal to provide the absolute pressure sensor. Patient's suffering from congestive heart failure are candidates for such implantable hemodynamic monitors. Such patient's suffer from episodes of cardiac insufficiency accompanied by labored breathing that is not presently monitored.
The detection of heart sounds rather than the R-wave peak or a pressure sensor signal has also been suggested anonymously in RESEARCH DISCLOSURE No. 37150, entitled "Use of Heart Valve Sounds as Input to Cardiac Assist Devices" (March, 1995) for use in controlling operations of pulsatile cardiac assist devices. The listed cardiac assist devices include intra-atrial blood pumps (IABPs), cardiomyoplasty/cardiac assist devices (of the type described in commonly assigned U.S. Pat. No. 5,069,680, for example), aortomyoplasty and ventricular assist devices (VADs). The heart sounds are picked up by a microphone, amplified, bandpass filtered and compared to a "signature" sound pattern to derive a control signal timed to the second or "dub" heart sound, which is related to the dicrotic notch of the aortic pressure wave. No specific structure for accomplishing this is disclosed. In U.S. Pat. No. 4,763,646 to Lekholm, a heart sound detector is also proposed to be mounted in one or more pacing leads arranged in or about the heart or to be mounted in the IPG case for acoustically sensing heart sounds transmitted through a fluid filled lumen. The use of a pressure sensor, microphone or accelerometer is proposed for the heart sound detector.
Despite the prodigious effort expended in developing such indwelling sensors, few have been found to be useful or acceptable for chronic use. Typically, the foreign object body reaction encapsulates the sensor and isolates it from the parameter to be detected and measured. In addition, the active sensors require additional lead conductors and are electrically inefficient. Consequently, such sensors are complex in design, difficult to manufacture, relatively expensive, and short lived.
For example, a great deal of effort has been expended in developing indwelling absolute pressure and pressure rate of change sensors as noted above for measuring these parameters in a heart vessel or chamber. Many designs of such chronically or permanently implantable pressure sensors have been placed in limited clinical use. Piezoelectric crystal or piezoresistive pressure transducers mounted at or near the distal tips of pacing leads, for pacing applications, or catheters for monitoring applications, are described in U.S. Pat. Nos. 4,023,562, 4,407,296, 4,432,372, 4,485,813, 4,858,615, 4,967,755, and 5,324,326, and in PCT Publication No. WO 94/13200, for example. These sensors and an improved sensor and operating system are described in detail in commonly assigned U.S. patent application Ser. No. 08/394870 filed Feb. 27, 1995, for IMPLANTABLE CAPACITIVE ABSOLUTE PRESSURE AND TEMPERATURE SENSOR and Serial No. 08/394,860 filed Feb. 27, 1995, for IMPLANTABLE CAPACITIVE ABSOLUTE PRESSURE AND TEMPERATURE MONITOR SYSTEM.
Other semiconductor sensors employ CMOS IC technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced from stationary plates. The change in capacitance due to pressure waves acting on the diaphragm is measured, typically through a bridge circuit, as disclosed, for example, in the article "A Design of Capacitive Pressure Transducer" by Ko et al., in IEEE Proc. Symp. Biosensors, 1984, p.32. Again, fabrication for long term implantation and stability is complicated and unproven.
This concentrated focus on the development of accurate, efficient, reliable and permanently implantable, indwelling pressure sensors on lead bodies has taken place in recognition that the detection of pressure waves, particularly in the cardiac and pleural context, has a great number of applications as described above. However, the indwelling pressure sensor approach requires the implantation of the sensor into the heart chamber, where certain sensor types may become fibrosed and lose the ability to respond to mechanical heart motion or blood pressure changes associated with the contraction of the heart chamber or in association with the lungs or diaphragm where fibrosis may also render it ineffective.
In virtually all approaches, it is necessary to rely on additional components and circuitry which consume more energy, require additional lead conductors, increase the bulk and cost of the system, increase the cost of the implantation, and raise reliability issues. The additional components and circuitry are increased further in dual chamber pacemakers. Very few of the numerous approaches of the prior art have been attempted in an implantable pacemaker system and fewer yet have been proven clinically useful and commercially successful.
Therefore, despite the considerable effort that has been expended in designing such sensors, a need exists for a body implantable, durable, long-lived and low power sensor for accurately sensing pressure waves and related parameters in the body over many years of implantation.